The present invention relates to the production and characterization of Human Immunodeficiency Virus type 2 gp160. The virus has been demonstrated to have infected humans from a number of geographical areas of the world, and it has been further shown that HIV-2 infected individuals develop the disease characteristics similar to that of Acquired Immunodeficiency Syndrome (AIDS) caused by HIV-1. The course of disease progression in HIV-2 infected individuals is significantly slower than that of the pathogenesis caused by HIV-1.
HIV-2 is T-cell tropic just as HIV-1 is, and the CD4 antigen on the surface of T-cells acts as the virus receptor. The interaction of the viral envelope with the CD4 antigen initiates the virus infection process. The envelope glycoprotein of HIV-2 is synthesized in the virus infected cells as a precursor gp160, which is subsequently cleaved into the external envelope glycoprotein gp120 and the transmembrane gp41 protein before being packaged in the virus envelope. Unlike the gp41 in HIV-1, the transmembrane protein is present as intact gp41 or as truncated gp31 in the HIV-2 virions. Further, the HIV-2 virions with the truncated gp31 are highly infectious while the corresponding defective HIV-1 is non-infectious. This makes the biological and biochemical properties of the envelope proteins not readily comparable.
In view of their location, the envelope proteins gp120 and gp41 are the primary targets of immune response in HIV-2 infected humans. This is similar to the immune response to HIV-1 in HIV-1 infected individuals. However, the two viruses have less than 50% homology in their envelope proteins and hence it is difficult to extend the observations made in the HIV-1 system to the responses to HIV-2. For example, at least in animal model systems the primary target of the immune response is the V3 loop of the gp120 in the HIV-1 system, and this is true among very divergent HIV-1 isolates. On the other hand no such principal immune domain has been identified in the HIV-2 envelope protein. However, the one similarity between the two viruses is the extensive variability in the envelope glycoproteins among multiple isolates of both HIV-1 and HIV-2. This has limited the use of subunits of the gp120 or gp41 of HIV-2 for either diagnostic use or as candidate vaccines in humans.
The ability to obtain certain human T-cell lines chronically infected with defective forms of HIV-1, which secrete intact gp160, has been described in the U.S. Pat. Nos. 5,116,740 and 5,122,468. However, the ability to obtain such cell lines is not universally applicable to all variants of HIV-1. The case with HIV-2 is even more difficult to predict than that of HIV-1, as HIV-2 is far less cytopathic than HIV-1 and HIV-2 is still infectious with the truncated form of the gp31. Also, the development of either HIV-1 or HIV-2 infected cell lines secreting the envelope precursor protein gp160 is not generally applicable to all the diverse variants of these two viruses. Experience has demonstrated that each individual virus isolate is unique and requires selective handling and different manipulations of the culture system, which may still not yield success. The defining properties of the virus which ultimately lead to the secretory phenomenon are not yet clearly understood. For example, it has been repeatedly tried without success to develop cell lines secreting gp160 from a number of isolates, including another variant of HIV-2, HIV-2 SBL.sub.6669. The development of the 6D5.sub.NIHZ of the present invention took more than three years of developmental work with no clear chance of success, before a cell line was obtained which secreted HIV-2 gp160 in quantities suitable for large-scale isolation.
With respect to the HIV-1 group of viruses, during the last seven years of research and development, success has been reached only with the HIV-1.sub.IIIB isolate and the 6D5.sub.451 system set forth in U.S. Pat. Nos. 5,116,740 and 5,122,468. It has not been possible, after years of research, to develop a gp160 secreting cell line from most HIV isolates, including HIV-1.sub.MN, HIV-1.sub.CDC71, HIV-1.sub.WOM, HIV-1.sub.Z6, and HIV-1.sub.Bal. It has also been attempted, but has repeatedly resulted in failure, to develop a cell line of SIV.sub.mac251 infected cells which secrete SIV.sub.mac251 gp160 (SIV.sub.mac251 being clearly related to HIV-2 and having &gt;70% homology in the envelope protein). In view of the uniqueness of each virus, and of each individual virus strain, as well as in view of the very few instances of being able to develop a cell line which secretes functionally intact gp160, the chance of success in developing any virus infected cell line which secretes gp160, and in particular the likelihood of developing a cell line which secretes HIV-2 gp160, functionally intact, is low.
Until the present invention, the gp160 of HIV-2 has been produced only by recombinant techniques. However, the glycosylation of the recombinant protein varies significantly, both qualitatively and quantitatively, from the native protein from HIV-2 infected cells. Because of this, there are differences in the conformation between gp160 from natural viral culture and the recombinant gp160. These have become critical issues for diagnostic and vaccine strategies against AIDS.